Which aircraft is more difficult to be selected for as a young officer: Apache or Wildcat?

Thank you for your questions.

Selection for an aircraft type is not based on difficulty. Being a qualified aviator whether on a rotary or fixed-wing aircraft is challenging in itself, especially when you take into account the management of the aircraft systems and also potentially 'fighting' with the aircraft in an operational environment.
Essentially each aircraft type (Wildcraft/Apache) brings its own different set of challenges and rewards.

Preference of aircraft type takes place during the Army pilots course. In loose terms, the individual (potential pilot) will be invited to choose not one, but multiple preferences of aircraft types and place them in priority order. I would highlight here that this is only a personal preference and whilst the Army will try to honour this preference, the needs of the service will come first (number of qualified pilots to aircraft type where required).

As for the Gazelle helicopter, this aircraft has served with the Army Air Corps since 1975 and has seen service during some fundamental periods in history such as the Falklands War, the fall of the Berlin wall, Bosnia and Northern Ireland and more recently, Iraq; a service spanning some 40+ years.
With this in mind the aircraft is now finally coming to the end of its service and will be retiring from the Army Air Corps very soon. Training on the Gazelle will unfortunately no longer be an option. Who knows, there may (one day) be a replacement for it???

Hope this answers the question(s).